Method of unloading two stage compressors



Dec. 25., 1962 w. GRANT ETAL METHOD OF UNLOADING TWO STAGE COMPRESSORS Filed July 21, 1959 United States Patent 3,070,282 METHOD OF UNLOADING TWO STAGE COMPRESOR Whitney 1. Grant, Muskego, and John W. Deroche, Milwaukee, Wis, assfgnors, by mesne assignments, to Vilter Manufacturing Corporation, a corporation of Wisconsin Filed .luly 21, 1959, Ser. No. 828,629 3 Claims. (Cl. 230-26) The invention relates in general to improvements in the art of controlling the operation of compressors, and it relates more particularly to an improved method of unloading plural stage gas compressors.

The primary object of the present invention is to provide a method of insuring positive and rapid loading and unloading of all stages of a plural stage gas compressor.

It is common knowledge that a two stage gas compressor, especially of the type used in large refrigeration installations, usually comprises high and low pressure cylinders or stages built into a common compressor frame. This frame is partitioned in order to provide segregated suction, intermediate, and high pressure chamhers, and the crankcase chamber is normally separated from both the low and high pressure suction chambers by a common partition and is maintained at the intermediate pressure which is the low pressure stage discharge and the high pressure stage inlet pressure. The refrigerant vapor from the evaporator is initially compressed in the low pressure stage and after passing through an intercooler or de-superheater is ultimately compressed in the high stage and discharged at the final high pressure required.

Such two stage compressor is usually provided with a capacity control arrangement consisting of an unloading device applied to the high and low pressure cylinders which are intended to effect the desired capacity controls, and each of these unloaders comprises a plunger for lifting the intake valve of its compression cylinder and which is operable by compressed gas discharged by the corresponding stage. When a compressor thus equipped is started, it requires several revolutions of the piston actuating crankshaft before sufiicient gas discharge pressure has been produced to actuate the unloader plungers so as to close the suction valves. The pressure difference across the plungers which operate the low and high pressure stage unloaders will then vary considerably because the pressure acting upon the lower face of the low pressure unloader plunger when vertically movable is equivalent to the evaporator suction pressure while that acting upon the corresponding lower face of the high pressure unloader plunger when likewise movable is equivalent to that in the intermediate chamber, whereas, the pressure acting upon the opposite faces of both of these plungers is equal to the final compressor discharge pressure. This condition results in the application of a greater force on the low pressure stage unloader plunger as compared to that on the high pressure stage unloader plunger, and the low pressure cylinders will therefore become loaded in advance of the high pressure stage and when this happens the intermediate pressure immediately rises thus further reducing the pressure difference across the high pressure stage plunger and preventing this stage from loading. Operice ati-on with gas pressure alone under such conditions results in over-loading of the low pressure stages, production of excessively high temperatures, and premature compressor failure.

While it has heretofore been proposed to provide orifices and gas line restrictors in booster banks unloaders in order to cause the high pressure stage to load first,

this method is not sufficiently reliable to be employedv in automatically functioning gas compressors. Then too, it has been proposed to provide a diiferential pressure switch in order to delay deenergization of the unloading solenoid actuated valves and loading of the compressor until the diiference between the discharge and intermediate pressures has been equal to or exceeds the differential required to actuate the unloaders, but this proposal is likewise not sufficiently adequate for reliable compressor operation under varying conditions existing during normal operation of such gas compressors, so that unloading by variations in gas pressures alone has not been satisfactory.

Although it has also been proposed to actuate the unloader plungers of all stages from unloaded to loaded position with the aid of oil under pressure, there is always a tendency for a small amount of such oil to leak past the plungers into the suction chamber. These compressors can be readily protected against such leakage of limited quantities of oil by providing check valves which operatewhen the crankcase and booster suction pressures are equal during inactivity of the compressor, so as to return the leakage oil to the crankcase chamber. But if the machine is'frequently loaded and unloaded during normal operation, the accumulation of such leakage oil builds up within the suction chamber and causes damage to the suction and discharge valves of the low pressure stages of the machine.

As to the high pressure stage of such a compressor, unloading with oil under pressure does not involve any special valve damage problem, because the pressures in the crankcase chamber and in the high stage suction chamber can readily be equalized by conducting leakage o-il back into the crankcase. But another problem is presented when oil pressure unloading is employed, whenever high low stage suction pressure exists whereupon it is impossible to unload the low pressure stages once they have been loaded. For example, if the suction pressure is 20 pounds per square inch gauge, while the intermediate pressure is pounds per square inch gauge, and the discharge pressure is pounds per square inch gauge, in order to unload the machine, a special valve must be provided which will drain oil into the crankcase and will equalize the pressures of the oil acting upon all of the unloader plungers thus applying crankcase pressures to all of these plungers. Since it requires approximately 30 pounds per square inch absolute pressure diiference to actuate the unloading plungers, it must be apparent that with such pressure differential existing between the suction and intermediate zones the low pressure stage will remain loaded. Operation of the unloader controls with oil pressure alone is therefore also unsatisfactory primarily due to excessive leakage oil accumulation in the booster suction chamber of the compressor and because of the extreme pressure diiferences existing in the several chambers.

It is therefore an important object of the present invention to provide an improved method of controlling the operation of a two-stage gas compressor by actuating the high pressure stage unloader with oil under pressure and the low pressure stage unloader with gas under pressure, whereby positive and most effective control of the machine is accomplished under all conditions of operation.

Another important object of the invention is to provide improved means for automatically loading and unloading the several stages of a plural stage compressor rapidly and accurately without danger of damaging valves or other parts of the machine.

Still another object of this invention is to provide an improved system for actuating the unloader units of a two stage multiple cylinder refrigerant vapor compressor in a manner whereby minimum attention is required in order to maintain the machine in eificient operation.

These and other more specific objects and advantages of the invention will be apparent from the following description.

A clear conception of the manner in which the improved method may be commercially exploited in a typical gas or refrigerant vapor compressor, may be had by referring to the drawing accompanying and forming a part of this specification, in which like reference characters designate the same or similar parts in the various views. H p V,

FIG. 1 is a diagram of a typical two stage refrigerant compression system embodying a central bank of high pressure cylinders H and two adjacent banks of low pressure cylinders L all of which are normally associated with a common frame and are provided with separate fluid pressure actuated loading and unloading controls;

FIG. 2 is an enlarged diagram showing the high pressure stage capacity control unit in loading position;

FIG. 3 is a similarly enlarged diagram of the unit shown in FIG. 2, but in high pressure stage unloading position;

FIG. 4 is a likewise enlarged diagram showing the low pressure stage capacity control unit in loading position;

FIG. 5 is a similarly enlarged diagram of the unit shown in FIG. 4, but in low pressure stage unloading position; and

FIG. 6 is a schematic illustration of a typical suction valve unloading assemblage for the high and low pressure stages of a compound gas compressor.

While the invention has been described herein as being especially and advantageously applicable to two stage gas compressors of relatively large capacity such as employed in refrigeration systems, it is not intended to confine the improvement to such usage; and it is also contemplated that specific descriptive terms employed herein be given the broadest possible interpretation consistent with the actual disclosure.

Referring to the several diagrams, the central bank H embodying one or more high pressure cylinders constitutes the high pressure stage, while the two similar banks L each embodying one or more low pressure cylinders constitute the low pressure stages of the compressor. The high pressure stage of the compressor is provided with a loading and unloading unit 9 cooperating with a cylinder of the bank H, while each low pressure stage is provided with a similar loading and unloading unit 10 cooperating with a cylinder of each bank L. These units 9, 10 are of well known construction comprising a plunger 13, 13' movable in one direction by fluid pressure within a casing and in the opposite direction by spring pressure adapted to lift and open the suction valve 22 of the compressor cylinder with which the unit is associated in order to unload that cylinder. As shown in FIG. 6, the plunger 13 of the unloader coacts with the swinging end of a lever 23 the opposite end of which has a fixed fulcrum pin 24, to close the suction valve 22 through a yoke 25 and pins 26 attached to this valve whenever the plunger 13 moves down; and the valve 22 is opened by one or more springs '27 coacting with the yoke 25 when the plunger moves up.

In accordance with the present invention, high pressure gas is normally obtained from the discharge chamber of the high pressure stage or cylinder bank H through a connection or orifice nipple 11 and is transmitted through both branches of a pipe line 12 to the tops of unloader plungers 13 in each of the two low pressure banks L through connections 14. A solenoid valve 15 is electrically deenergized and closed so as to allow pressure to build up in the line 12 in order to actuate the low pressure cylinder unloading plunger 13' and to thereby load the low pressure stages.

High pressure lubricating oil is obtained from the usual oil pump through a fitting 17 and a pipe line 16, and is applied through another solenoid valve 18 which is electrically energized and adapted to establish communication with the top of the high pressure cylinder unloader plunger 13. The oil pressure in the pipe line 16 must be maintained above the crankcase pressure in order to prevent the compressor from stopping due to the operation of the oil pressure failure switch. Because the lower face of the high pressure cylinder unloader plunger 13 is sub jected to crankcase pressure, there will always be a higher pressure existing at the top of this plunger no matter what the absolute value of the intermediate pressure may be. Positive loading of the high pressure cylinder is therefore providedat all times as long as the oil pressure is maintained, so that with this system there is no danger of having the compressor operating with the low pressure eylinders L loaded and the high pressure cylinders H unloaded, or vice versa. r

The compressor capacity reduction may be controlled by means of a remote control switch the function of which is to properly energiZe and deenergize the solenoids of the valves 15, 18 and 19. To unload the compressor this capacity control switch should energize both solenoid valves 15, 19 so as to open these valves, and to simultaneously deenergize the solenoid valve 18 so as to close this valve. As the pressure within the pipe line 12 is bled off and reduced through the valve 15 by delivery of gas into the low pressure cylinder suction chamber through the connection 20, the low pressure stage will be unloaded; and the high pressure cylinder unloader will actuate as the oil pressure in the pipe line 16 is bled off and reduced through the solenoid valve 19 into the crankcase past a connection 21.

To load the compressor the reverse action of the above mentioned solenoid valves takes place. Solenoid valves 15 and 19 are deenergized so as to close these valves, and to simultaneously energize solenoid valve 18 so as to open this valve. This allows high pressure gas to flow through line 12 thus loading the low stage cylinders and causing oil to flow through line 16 thus loading the high pressure cylinder.

It will therefore be apparent that the present invention in fact provides an automatic capacity control system for two stage gas compressors, which insures positive loading and unloading of both the low and high pressure cylinders with utmost reliability and precision. The improved result is obtained with the aid of a combination of discharge gas and oil under pressure applied to the several stages, and the present improved method has overcome the various defects of prior similar systems and has proven highly satisfactory and successful in actual use.

It should be understood that it is not desired to limit the invention to the improved steps as specifically applied to refrigerant vapor compressors, since obvious modifications within the scope of the appended claims may occur to persons skilled in the art.

We claim:

1. The method of unloading a compressor having high and low pressure stages, which comprises, unloading the high pressure stagewith liquid under pressure and the low pressure stage with gas under pressure,

2. The method of unloading a compressor having high and low pressure stages, which comprises, unloading the high pressure stage with liquid under pressure and the low pressure stage with gas under pressure derived from the discharge of the high pressure stage.

3. The method of unloading a compressor having high and low pressure stages, which comprises, unloading the high pressure stage only with oil under pressure derived from an outside source, and simultaneously unloading the low pressure stage only with gas under pressure derived from the discharge of the high pressure stage.

References Cited in the file of this patent UNITED STATES PATENTS 

